A conventional differential amplifier comprises a matched pair of bipolar transistors, a matched pair of load impedances, and a current source. Each load impedance is connected between a voltage supply and a collector of a respective one of the transistors, and emitters of both transistors are connected to the current source. Differential inputs are applied to bases of the transistors, and differential outputs are taken from the collectors of the transistors.
Two such differential amplifiers can be cross-coupled to construct a quadrature phase Voltage Controlled Oscillator (VCO) which oscillates with ninety degree phase shift across each differential amplifier and with one hundred and eighty degree phase shift in the cross-coupled feedback loop.
Unfortunately, typical manufacturing process variations affect the oscillating frequency of such VCOs so that such VCOs must be tuned after manufacture if oscillation at a particular oscillation frequency is required. W. P. Robins proposes tuning by means of varactor diodes which may be coupled between the collectors and the matched transistors in each differential amplifier (see PHASE NOISE IN SIGNAL SOURCES - THEORY AND APPLICATIONS, Peter Peregrinus Limited). Unfortunately, this method requires pretrimming of the load impedances to center the oscillation frequency because the tuning range achievable with varactor diodes alone cannot cope with circuit parameter variations which result from typical manufacturing process variations. Moreover, the resulting VCO typically has a fairly low quality factor (Q) due to non-linearities in the circuit parameters.
In Applicant's U.S. Pat. No. 5,185,581, referred to hereinbefore, there is disclosed a differential amplifier which can be used to construct voltage controlled oscillators and other high frequency resonant circuits while providing a tuning range which is sufficient to cope with circuit parameter variations which result from typical manufacturing process variations. In U.S. Pat. No. 5,185,581 the phase shift of the differential amplifier is controlled by the emitter current density of the differential pair. A major disadvantage of this approach is that the gain of the amplifier and consequently the power output of the oscillator are also controlled by the emitter current density.
In a paper entitled DESIGN CONSIDERATIONS IN HIGH-FREQUENCY CMOS TRANSCONDUCTANCE AMPLIFIER CAPACITOR (TAC) FILTERS by F. Krummenacher, 1989 IEEE International Symposium on Circuits and Systems, one approach of tuning the frequency and Q factor of gyrator-based resonators is disclosed. This approach, applicable with MOS devices, works at frequencies of up to 100 MHz and yields a reported gyrator Q factor of about 10.